This lesson introduces two critical methods in pharmacokinetics, the Wagner-Nelson and Loo-Riegelman methods, used for estimating the absorption rate constant (ka) for drugs administered via non-intravenous routes. The Wagner-Nelson method relates ka to the plasma concentration derived from the slope of a semilog percent unabsorbed time plot. However, it is limited to drugs with one-compartment kinetics and can be impacted by factors like gastrointestinal motility or enzymatic degradation.
On the other hand, the Loo-Riegelman method estimates ka by comparing plasma concentration-time profiles following different administration routes. It provides more comprehensive data, even for drugs with multicompartment characteristics, and aids in understanding drugs' relative bioavailability and absorption characteristics. Yet, it also has limitations, such as the concentration versus time data requirement for both oral and IV drug administration of the same subject and intra-subject between oral and IV administration studies.
From Chapter 7:
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